A fixing device employing a heat fixing method has been conventionally and commonly used as a fixing device for use in an electrophotographic image forming apparatus which image forming apparatus is often employed in a device such as a copying machine, a laser printer, or a facsimile. Among heat fixing methods, a roller fixing method using a fixing roller is often employed.
According to the roller fixing method, the fixing is carried out as follows. Namely, while a fixing roller and a pressure roller are being pressed against each other, a recording material on which an unfixed toner image is formed is carried to a fixing nip area between the fixing roller and the pressure roller so that toner is heated and melted. This causes the toner to be melted and fixed onto the recording material. Note that each of the fixing roller and the pressure roller is configured so as to house a heater serving as a heat source and so that its outer surfaces is coated with a material such as rubber having a high releasability or resin having a high releasability. According to the roller fixing method, the fixing roller is entirely retained at a predetermined temperature. Therefore, the roller fixing method is suitable for high-speed processing.
In recent years, full-color image forming apparatuses such as full-color laser printers have been widely used.
In general, a full-color image forming apparatus uses toner of the following four colors: magenta, yellow, cyan, and black. In order that the full-color image forming apparatus fixes a full-color toner image, it is necessary to carry out a color mixture of toner of a plurality of colors while the toner is being melted, unlike the fixing of monochrome toner in which fixing the monochrome toner is merely softened and fixed while being pressurized. Therefore, a fixing device for use in the full-color image forming apparatus has to completely melt the toner.
In the case of a fixing device employing a roller fixing method for use in the full-color image forming apparatus, a fixing roller is formed as below. Namely, an elastic body which is a rubber layer made from a material such as silicon rubber is provided on a support made from a material such as a metal having a good heat conductivity. Further, the elastic body is coated with a fluorocarbon resin having a good releasability.
According to the fixing device, the fixing roller with the rubber layer having a low heat conductivity has to be heated to a predetermined temperature at the start-up of the full-color image forming apparatus. Accordingly, an arrangement in which the fixing roller is heated only by a heat source, such as a heater, provided inside the fixing roller has a problem in that it takes a long time period from the power on the full-color image forming apparatus until the full-color image forming apparatus becomes ready for operation. Another problem is that a temperature of the fixing roller decreases during a high-speed continuous operation of the fixing device.
As a solution to the problems, a fixing device employing an external heating and fixing method has been proposed recently. In the fixing device, a belt member (external heating belt) having a small heat capacity is heated and is made to contact with a surface of the fixing roller. This causes the fixing roller to be heated not only from within but also externally.
Patent Literature 1 proposes a control method which gives higher priority to external heating. This is a method for further reducing a warm-up period of a fixing device employing an external heating and fixing method.
In the control method which gives high priority to the external heating, first, only a heat source of an external heating belt is turned on so that the external heating belt is heated to a predetermined first temperature. Thereafter, the heat source of the external heating belt is turned off, and a heat source inside a fixing roller is turned on. When a temperature of the external heating belt decreases to the predetermined first temperature or lower, the heat source inside the fixing roller is turned off, and the heat source of the external heating belt is turned on again. Then, the heat source of the external heating belt is turned off when a temperature of the external heating belt reaches the predetermined first temperature, and the heat source inside the fixing roller is turned on again. This is repeated until the fixing roller reaches a predetermined warm-up completion temperature.
Patent Literature 2 discloses a technique in which a fixing roller is realized by a hard roller and a pressure roller is realized by a soft roller, so that each linear velocity of the fixing roller and the pressure roller is changed during an envelope mode. This can prevent a recording material such as an envelope from being wrinkled while it is being subjected to a fixing process by a fixing device employing a roller fixing method.
Citation List
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2007-241143 A (Publication Date: Sep. 20, 2007)
Patent Literature 2
Japanese Patent Application Publication, Tokukai, No. 2004-205620 A (Publication Date: Jul. 22, 2004)